1. Field of the Invention
The present invention relates to a multilayer ceramic capacitor which has a large number of laminated layers of dielectric layers and inner electrodes, whose dielectric layers are each set to be thin, and which is compact in size yet large in capacitance.
2. Description of the Related Art
A multilayer ceramic capacitor is composed of a chip-shaped element assembly and a pair of outer electrodes which are formed at both end portions of the element assembly. The element assembly is in turn composed of a number of laminated layers in which a number of dielectric layers and inner electrodes are alternately laminated on one another. The inner electrodes are configured such that the adjacent electrodes are disposed so as to face each other with the dielectric layer between and to electrically connect them individually to the outer electrodes.
The element assembly may be produced by laminating ceramic green sheets and conductive patterns alternately on one another to form a laminated assembly in a chip shape and firing the chip-shaped laminated assembly at a high temperature in the range of approximately 1,200.degree. C. to 1,300.degree. C. in air.
The ceramic green sheet to be used for conventional laminated ceramic capacitors is composed of ceramic grain of, e.g., BaTiO.sub.3 type, etc., which has a high dielectric constant, and an organic binder as main components. On the other hand, the conductive pattern may be composed of a conductive paste containing powders of, e.g., Pd or the like as a main component.
The component such as Pd or the like to be used for the main component of the conductive paste is a noble metal which is expensive. Therefore, conventional multilayer ceramic capacitors are expensive in production costs. Recently, a base metal such as Ni or the like has been used as the main component for the conductive paste in order to reduce production costs of ceramic capacitors.
However, the use of the base metal such as Ni, etc., as the main component of the conductive paste for the production of chip-shaped laminated assembly may suffer from the disadvantages that the such base metal is caused to undergo oxidation, when it is burned at high temperature in air in a conventional way, and as a result that the conductivity of the inner electrodes may be lost. On the other hand, when the chip-shaped laminated assembly is burned at high temperature in a non-oxidative atmosphere in order for the base metal such as Ni or the like to fail to undergo oxidation, the dielectric layers are caused to be reduced, thereby lowering resistance to insulation and as a consequence failing to gain a desired level of electrical characteristics.
Therefore, when the base metal such as, e.g., Ni or the like is used as a material for the inner electrode, a desired level of electrical characteristics is designed to be achieved by using a material having a high degree of resistance to reduction for the dielectric layer. In this case, the firing is first carried out in a reductive atmosphere and the thermal treatment is then carried out at temperature lower than the firing temperature, i.e. at approximately 600.degree. C. to 900.degree. C., in an atmosphere containing a small amount of oxygen to re-oxidize the dielectric layer and recover the resistance to insulation of the dielectric layer, while preventing the inner electrode from undergoing oxidation.
Recently, there are growing demands to make multilayer ceramic capacitors compact in size and larger in capacitance in order to comply with requirements for electric circuits which have smaller in size yet higher in density. Therefore, attempts have been made to further increase a number of laminated dielectric layers and make them thinner, in order to meet such requirements for making the multilayer ceramic capacitor compact in size and larger in capacitance.
It has to be noted, however, that the further increase of the number of the multilayer dielectric layers and the thinning of them can improve a capacitance of the resulting multilayer ceramic capacitor, but they may cause a decrease in resistance to insulation of the resulting multilayer ceramic capacitor. The reason for the decrease in resistance to insulation is because the insulation resistance R can be represented by R=(.rho..times.d)/S, where R is resistance, .rho. is resistivity, S is the area of the electrode, and d is the thickness of the dielectric layer.
Further, a CR product can be enumerated as one item for characteristics of a capacitor. The CR product is the product obtained by multiplying the capacitance C by the insulation resistance R. Thus, as the capacitance C can be represented as C=.epsilon.o.times..epsilon.r.times.S/d, wherein .epsilon.o is a dielectric constant in vacuo; and .epsilon.r is a specific dielectric constant, the CR product can be represented by C.times.R=.rho..times..epsilon.o.times..epsilon.r. As a result, it can be found that the CR product can give a value that does not depend upon the thickness of the dielectric layer and the number of the layers thereof.
It should be noted, however, that, generally speaking, the CR product shows the tendency that it may decrease when the thickness of the dielectric layer becomes thinner than 5 .mu.m. This is considered to occur because the resistance to insulation is caused to deviate from the ohmic rule as the dielectric layer becomes thinner. In other words, ceramic capacitors may cause the problem that the CR product may be reduced as the ceramic capacitor becomes thinner.